For many years, lawn mower blades were simply bolted on to the bottom of a rotating shaft. As the blades themselves were a little more than straight pieces of sharpened stamped steel, and as all of the forces on the blade were unidirectional, this method of interconnection worked quite well.
When the requirements for passing torque between the blade and shaft became bidirectional, as with the addition of a blade brake to the interconnection, and as the blades themselves began to require greater torque, for example mulching blades having integral fans and recutting sections, the simple bolt no longer sufficed. Manufacturers, therefore, adapted any one of many varying techniques in order to physically connect the blade normally to some sort of adapter between the blade and the shaft. These included adapters having pins spaced from the rotational axis of the shaft as well as star shaped holes in the blade with corresponding keys on the shaft.